First of all you should avoid Fusion drive at all cost, it is not suitable for Pro Audio. Period.
Second, go to
www.everymac.com and see the specs of the models you are looking for. There are geekbench numbers available for every CPU out there and you can have a good idea how the models perform in comparison to each others. Bear in mind that low single-core performance is not necessarily a problem unless you run one very CPU-hungry process (plugin for example) but you generally speaking want to get good multi-core performance.